How many Recommendation letters should I have?

I am upgrading my CV and preparing recommendation letters to join a Phd program, I've got one reference letter from my Prof.(he is abroad now) and my former director(joined another univ.) Should I have more than two letters?

however sometimes you are only asked to give referees who can be contacted by the selection board instead reference letters. If asked to do so: ask the people you name first if they are happy to be on your list of references - because we currently had a case where these references did not know the student besides from a 2day practical course - and of course this student was immediatly removed from the competition for the job.